As a new type of nonvolatile storage technology, a resistive random access memory (RRAM) and a phase change memory (PCM) with advantages of high storage density, low power consumption, fast read and write speeds, long-time retention, multiple-bit storage, small cell area, compatibility with a complementary metal oxide semiconductor (CMOS) technology, etc. have drawn more attention. Particularly, RRAM and PCM which may realize three-dimensional integration are becoming a hot research topic of high-density memory device, and a multi-layer stacked crossbar structure is the most attractive structure. However, the multi-layer stacked structure of a three-dimensional memory device may cause problems such as disturbance between memory cells in a same layer or between different layers, leakage current and fabrication difficulty.
Currently, a one transistor one resistor (1T1R) structure and a one diode one resistor (1D1R) structure are the most popular cell structures for a three-dimensional RRAM and a three-dimensional PCM. The 1T1R structure means that one MOS transistor is connected in series with one variable resistor for selecting or isolating the variable resistor. However, the transistor as an active device needs to be fabricated in a front-end procedure, and a minimum cell area restricted by the transistor is unfavorable to high-density three-dimensional stacking of the memory devices. The 1D1R structure means that one diode is connected in series with one variable resistor, in which selecting the resistor is realized by a rectification property of the diode. The diode properties such as high forward current density, high on/off current ratio and processing compatibility are important selection criteria. Although a diode based on monocrystalline silicon has high current density and high rectification ratio, the diode needs high processing temperature and is difficult to fabricate on a metal electrode. Although a diode based on a metal oxide has good processing compatibility, the forward current density of the diode is not ideal.
Therefore, there is a need for a RRAM or a PCM based on a new type of selection transistor that has simple memory cell structure with independence on a silicon substrate, multi-layer stacked structure and three-dimensional high-density storage.